Current Fluctuations of the One Dimensional Symmetric Simple Exclusion Process with Step Initial Condition

For the symmetric simple exclusion process on an infinite line, we calculate exactly the fluctuations of the integrated current Q(t) during time t through the origin when, in the initial condition, the sites are occupied with density rho(a) on the negative axis and with density rho(b) on the positive axis. All the cumulants of Q(t) grow like root t. In the range where Q(t) similar to root t, the decay exp[-Q(t)(3)/t] of the distribution of Q(t) is non-Gaussian. Our results are obtained using the Bethe ansatz and several identities derived recently by Tracy and Widom for exclusion processes on the infinite line.